1. Field of the Invention
The present invention relates to a device having an element (hereinafter referred to as luminous element) that is comprised of a luminous material sandwiched between electrodes (the device will hereinafter be referred to as light-emitting device), and to a method of manufacturing the same. Specifically, the present invention relates to a light-emitting device using a luminous material that provides EL (Electro Luminescence) (hereinafter referred to as EL material).
2. Description of the Related Art
In recent years, development is proceeding in a light-emitting device (EL display device) using a luminous element that utilizes the EL phenomenon of a luminous material (hereinafter referred to as EL element). The EL display device is a display device that uses a luminous element which itself has a light-emitting ability, that is, self-emissive, and hence, unlike a liquid crystal display device, does not need a back light. In addition, the EL display device has merits such as a wide field of vision, light weight, and a low power consumption.
Such an El display device is constructed of a structure that has an EL element composed of an anode, a cathode, and an EL material sandwiched therebetween. By applying a voltage between the anode and the cathode to make a current flow in the EL material, carriers re-couple, whereby the EL element emits light. Such a driving method is called a current drive. However, in the EL display device which has the current drive, there is a problem of a phenomenon in which a voltage drop (also referred to as an IR drop) caused by a wiring resistance occurs. This phenomenon is that the voltage becomes lower as its distance from a power source becomes farther even if it is the voltage of the same wiring. This problem is particularly conspicuous when the wiring becomes long. Thus, it is a large obstacle in making the screen of the EL display device larger.
When the wiring is made of a material such as tantalum, tungsten, or silicon, the EL display device is susceptible to the influence of the wiring resistance, which may become the cause of immensely reducing the homogeneity of the quality of the image. In addition, in case of using a low resistant material such as aluminum or copper, when the draw-around distance is long, then the same thing can be observed, i.e., the aforementioned phenomenon will occur.
The above-mentioned problem will be explained here with reference to FIG. 2. Shown in FIG. 2 is a portion of a pixel portion of an active matrix EL display device. An n number of pixels denoted by A1, A2, . . . An are arranged in the up and down direction (vertical direction) of the diagram. Reference symbol 201 denotes a gate wiring, 202 denotes a source wiring, and 203 denotes a current supply line. Furthermore, a switching TFT 204, a storage capacitor 205, a current control TFT 206, and an EL element 207 are formed in a region that is surrounded by the gate wiring 201, the source wiring 202, and the current supply line 203.
At this point, the voltage of the current supply line 203 drops as the current supply line 203 moves towards the bottom of the diagram due to the influence of the voltage drop. That is, a voltage V1 that was in the upper part of the pixel portion becomes a voltage V2 in the lower part of the pixel portion, becoming a relationship of V1>V2. This influence becomes more conspicuous as the area of the pixel portion (image display region) is made larger.
As a result, in case of making the EL elements of each of the pixels emit light in the same brightness, the pixel denoted by A1 and the pixel denoted by A2 will emit light in about the same brightness. However, the brightness of the light emitted by the pixel denoted by An declines compared with the pixel denoted by A1 and the pixel denoted by A2. The reason for this originates in that the voltage applied to the EL element of the pixel denoted by An has declined due to the voltage drop.
Further, the influence of such voltage drop is imparted not only to the current supply line 203 but also to the gate wiring 201 and the source wiring 202. In other words, there is a concern that the gate wiring 201 may not be able to open a gate of the switching TFT 204 because of the voltage drop. In addition, the source wiring 202 becomes incapable of applying a desired voltage to a gate of the current control TFT 206 due to the voltage drop, leading to a fear that the brightness of the EL element will change or that the EL element will not emit light.
Thus, the transmission of a desired voltage becomes impossible because of the voltage drop which originates in the wiring resistance.
Consequently, a drawback in which there is a considerable loss in the homogeneity of the quality of the image in the pixel portion occurs. Attempts such as contriving to apply a voltage to both ends of the wiring have been made in order to improve the above problem. However, because the wiring is drawn around longer, as a result, the influence of the voltage drop cannot be ignored.
In case of manufacturing a monolithic type light-emitting device in which a driver circuit portion (typically including a gate driver circuit and a source driver circuit) is integrated on the same substrate, the wiring resistance of a wiring that is drawn around between the driver circuit portion and an input terminal of an electric signal becomes a problem. The wiring resistance induces a delay of the electric signal, and therefore there is a concern that the operating speed of the gate driver circuit and the source driver circuit will be reduced.
Thus, drawbacks such as the considerable loss of the homogeneity of the quality of the image and the extreme decline in the operating speed of the driver circuit portion due to the voltage drop which originates in the wiring resistance and the delay of a signal occur.
Such a problem becomes a particularly conspicuous problem in a light-emitting device having a large screen that is several tens inches in diagonal.